The present invention relates to a developing device for a multicolor image forming apparatus.
An electronic color copier and a digital color copier, for example, belong to a family of multicolor image forming apparatuses which sequentially form electrostatic latent images on a single image carrier and develop the latent images one after another by toners of different colors to produce a multicolor image. The word "multicolor image" covers a broad range of images, i.e., a full-color image to a bicolor image. Generally, a developing device incorporated in such an image forming apparatus has a plurality of developing units each storing a toner of particular color and sequentially arranged around the image carrier. The problem with the multicolor image forming apparatus is the mixture of colors ascribable to toners flying about. Let this problem be discussed taking a red toner and a black toner as an example. When a black toner develops a latent image, it is partly scattered around in the image forming apparatus to contaminate the interior of the apparatus. Part of the black toner scattered around enters a developing unit storing a red toner and is, therefore, mixed with the red toner. As the amount of black toner mixed with the red toner increases, the resulting red image is impure and appears brownish. This critically degrades the quality of the bicolor image.
In the light of the above, a brush for preventing a toner from flying about may be located at the inlet side and the outlet side of each developing unit, as proposed in the past. Here, the inlet side and the outlet side refer respectively to the side where a latent image formed on the image carrier arrives at the developing unit and the side where it leaves the developing unit. The tip of each brush is held in contact with the surface of the image carrier except for the range where the image area of the image carrier moves. Although the brushes successfully prevent the toner from flying about in the non-image area, it cannot do so in the image area. Moreover, the length of the bristles of each brush have to be sequentially increased in the intended direction of movement of the image carrier, increasing the production cost of the developing units and confusing the assembly of the device.
When a plurality of developing units constituting a multicolor developing device are arranged one above another, the mixture of colors is more conspicuous in a lower developing unit than an upper developing unit. This will be readily understood from the fact that the scattered toner falls due to gravity. It will also be seen that as the periphery of the image carrier moves, the mixture of colors is more aggravated in a downstream developing unit with respect to the moving direction of the image carrier since the image carrier generates a stream of air in the moving direction. Usually, the toner is adhered to a carrier by electrostatic attraction. As the electrostatic force exerted by the latent image at the developing station overcomes the electrostatic attraction, the toner is transferred to the image carrier to develop the latent image. Toner particles deposited on the carrier by a weak force, i.e., carrying a small charge are scattered around. The amount of charge differs from one toner particles to another and generally increases with the decrease in particle size. Therefore, considering the entire toner, the amount of charge increases with the decrease in average particle size.
Assume that the average particle size of toner is sequentially increased from an upstream developing unit toward a downstream developing unit with respect to the moving direction of the image carrier or from an upper developing unit toward a lower developing unit. Then, the amount of charge of toner sequentially decreases from the upstream or upper developing unit toward the downstream or lower developing unit. The toner particles flown out of the upstream or upper developing unit carry smaller charges than the other particles in the same unit. However, once introduced into the downstream or lower developing unit, the charges of such toner particles are great relative to those of the toner particles existing in the unit, causing them to strongly adhere to the carrier.
In the event of development, the toner particles weakly adhered to the carrier are consumed first. Since the toner particles introduced in the downstream or lower developing unit have the adhesion thereof to the carrier increased relative to the toner particles originally existing therein, they are consumed little in the unit. It follows that although the toner particles scattered around from the upstream or upper developing unit may enter the downstream or lower developing unit, the mixture of toners is effectively reduced when it comes to the resulting toner image.
The amount of charge tends to decrease with the increase in the particle size of toner, as stated earlier. Hence, the charge distribution of toner resembles the particle size distribution of toner. Specifically, when the particle size distribution has a broader skirt, so does the charge distribution; the amount of toner particles carrying small charges and, therefore, apt to fly about increases. By loading all the developing units with toners having the same particle size distribution and the same ratio of the volumetric average particle size to number average particle size, it is possible to reduce the toner particles whose adhesion to the carrier is weak and, therefore, the amount of toner to fly about.
Considering resolution, for example, the reproducibility of a toner image increases with the decrease in the particle size of toner. In this sense, if the average particle size of toner which differs from one developing unit to another is confined in a certain range, the toner images formed by the respective toners will not appear different in quality at least to the eyes.
The present invention is based on the above considerations.